Final answer:
To determine the amount of oxygen gas initially present, we calculated the mass of oxygen that reacted by subtracting the mass of combusted fuel from the combined mass of products. Converting this to moles and using Avogadro's number, we find approximately 4.10 x 10^21 molecules of O2 were initially present.
Step-by-step explanation:
The question asks about the amount of oxygen gas initially present in a combustion reaction where unknown fuel was burnt and produced water and carbon dioxide with some unreacted fuel left.
To solve this, we first need to find out the mass of fuel that reacted. That is the initial mass of the fuel (17.65 g) minus the remaining unreacted fuel (17.59 g), giving us 0.06 g of fuel combusted. Since oxygen was fully consumed, we can relate the mass of the products to the mass of the oxygen.
Given that the combined mass of water and carbon dioxide produced is 0.0981 g + 0.1797 g = 0.2778 g, and we know the mass of the combusted fuel, we can infer that the difference in mass (0.2778 g - 0.06 g) is due to the mass of oxygen (O2) that has reacted. Therefore, the mass of oxygen reacted is 0.2178 g.
We now convert this mass to moles using the molar mass of oxygen (32.00 g/mol for O2), resulting in 0.2178 g / 32.00 g/mol = 0.00680625 moles of O2. Since 1 mole of gas consists of Avogadro's number of molecules (6.022 x 1023 molecules/mol), we multiply the number of moles by Avogadro's number to find the number of molecules: 0.00680625 moles * 6.022 x 1023 molecules/mol ≈ 4.10 x 1021 molecules of O2.